The present invention relates to various improvements to the access and tracking device for an optical disc described in French Patent Application No. 89-08800 filed on 30 Jun. 1989 in the name of ART TECH GIGADISC "ATG".
This Patent Application describes a device for access to and tracking of a track on an information medium, the information being readable and/or recordable optically by a focused beam such as an optical disc, which exhibits the lightest and the most compact structure possible. For this reason, the elements of the access and tracking device such as the optical head and the two coils associated with the motor-devices permitting respectively access and radial tracking, as well as focusing, to be achieved are fixed rigidly to one another in such a way as to form a single compact mobile unit moving in the gaps in the magnetic circuits. Hence, as represented in FIG. 1, the access and tracking device described in French Patent No. 89-08800 essentially comprises at least one optical head 1 which will be described in more detail later, a first coil 2 for access and radial tracking and a second coil 3 for focusing. This assembly forms a compact unit. In fact, as represented in FIG. 1, the optical head 1 is constituted more particularly by a cylindrical element 1' carrying, at its upper end, a focusing lens 4 and, at its lower end, a return mirror 5 intended to reflect the beam given off by the laser source towards the optical disc, not shown. The axis A of the head is placed orthogonally to the plane of the disc. This head 1 is fixed on a rigid support element 6 which is integrated with the coil 2. This coil 2 can be moved in translation in the gap of a first fixed magnetic circuit. In order to simplify the drawing, only the central core 7 of the first fixed magnetic circuit has been represented. In a known way, this core 7 is secant to all the grooves or revolutions of the optical disc and extends over a length equal to at least the useful recording length augmented by the length of the mobile unit. This first magnetic circuit, in association with the coil 2, thus forms the first motor device permitting access and tracking of the track. Moreover, as represented in FIG. 1, below the coil 2 is fixed, by means of a fixing system such as brackets or rigid elements, not shown, a coil 3 used for focusing. This coil 3 can be moved in the gap of a second magnetic circuit, not shown in FIG. 1, which, in association with the coil 3, forms the second motor device permitting focusing. Moreover, as shown in FIG. 1, the mobile unit comprises an anti-pitching device 8 which consists, in the embodiment represented, of a slide 9 into which can slide a stem 10 fixed at one end to the mobile unit and bearing at its other end specific anti-pitching means whose shape corresponds to the shape of the slide 9.
As shown in FIGS. 1 or 2, the coil 2 may be constituted by a cylindrical frame with axis parallel to a radius of the disc onto which is wound at least one conductor whose turns are oriented, in their active part, along the optical axis of the head. In the embodiment represented in FIG. 1, the coil 3 is constituted by a flat coil of substantially elliptical shape. This coil comprises conductors oriented perpendicularly to the optical axis A of the optical head 1.
In order to simplify the mobile unit and render it more compact, French Patent Application No. 89-08800 also proposes a solution using only a single magnetic circuit to produce the two access and radial tracking as well as focusing devices. In this case, an access and radial tracking coil 2 is used, as well as a focusing coil 3' such as those represented in FIG. 2. The coil 2 is substantially identical to the coil 2 of FIG. 1. In this case, the coil 2 is a self-supporting coil obtained by cylindrically winding a conductor, the rigidity of the coil being provided solely by the bonding of the various turns. However, the coil 2 could also be produced by winding on a cylindrical former. The sides 2a and 2b represent the active parts of the conductor or of the conductors of the coil 2. The coil 3 is then constituted by a flat coil bent into a U in such a way as to be fitted under the access coil 2. In this case, the parts 3'a and 3'b are the active portions of the conductor or of the conductors of the coil 3'.
With the coils described above, only one magnetic circuit is in operation. However, this type of coil exhibits a certain number of drawbacks. The bonding of the two coils is difficult to produce. Moreover, as the lower coil is fixed to the coil 2 only by a film of adhesive, resonance phenomena are observed. Furthermore, in order to keep these resonances to a sufficiently-high frequency, aluminium wires have to be used, which are more problematic in use than copper wires.